教授给我的单元测试出了点小问题。编译时，我收到了以下错误：
cannot find symbol import org.junit.Assert.assertArrayEquals; cannot find symbol import org.junit.Assert.assertEquals; import org.junit.Assert.assertFalse; import org.junit.Assert.assertTrue;
我已经下载了JUnit，并且可以编译一个类似的文件，那么为什么我会遇到问题呢？代码是：

import java.util.Comparator;
import org.junit.Assert.assertArrayEquals;
import org.junit.Assert.assertEquals;
import org.junit.Assert.assertFalse;
import org.junit.Assert.assertTrue;
import org.junit.Before;
import org.junit.Test;
    public class SortingTests {
      class IntegerComparator implements Comparator<Integer> {
        @Override
        public int compare(Integer i1, Integer i2) {
          return i1.compareTo(i2);
        }
      }
      private Integer i1,i2,i3;
      private OrderedArray<Integer> orderedArray;
      @Before
      public void createOrderedArray(){
        i1 = -12;
        i2 = 0;
        i3 = 4;
        orderedArray = new OrderedArray<>(new IntegerComparator());
      }
      @Test
      public void testIsEmpty_zeroEl(){
        assertTrue(orderedArray.isEmpty());
      }
      @Test
      public void testIsEmpty_oneEl() throws Exception{
        orderedArray.add(i1);
        assertFalse(orderedArray.isEmpty());
      }
      @Test
      public void testSize_zeroEl() throws Exception{
        assertEquals(0,orderedArray.size());
      }
    }